Conventional internal combustion engines such as diesel or spark-ignited engines require controlling of combustion quality features such as the SOC for both efficiency and emissions control. For example, the diesel engine controls the start of combustion by the timing of fuel injection while a spark-ignited engine controls the start of combustion by the spark timing.
However, additional combustion control may be desirable on conventional engines and required on other engines. An example of an engine that benefits strongly from a feedback combustion quality sensor is one that runs using HCCI type combustion. Unlike a traditional SI or diesel engine, HCCI combustion takes place spontaneously and, in general, homogeneously without flame propagation. HCCI combustion is the compression ignition of a relatively well-premixed fuel/air mixture. Various combustion strategies based on the HCCI principle have been developed. For example, the combination of an HCCI engine with traditional engine injection technology led to Premixed Charge Compression Ignition (PCCI) where the fuel/air mixture is premixed, but not necessarily homogeneous. An additional combustion strategy is a strategy that supplements the energy provided by a PCCI combustion event with a directly injected quantity of fuel generally provided once combustion has commenced. This type of engine is known as a premixed charge direct injection (PCDI) engine.
One problem with HCCI engines is that the combustion quality is sensitive to a large number of parameters including intake manifold temperature, fuel/air ratio, fuel quality, trapped residual gas fraction and exhaust gas recirculation quantity, amongst others. Without control over the parameters that impact combustion quality, large cycle-to-cycle variations in combustion quality will be encountered. Thus, in as compared to conventional diesel and SI engines, misfires and excessively fast rates of pressure rise are apt to occur. Knowledge of the time of commencement of combustion, that is SOC or, as it is sometimes referred to, combustion phasing, can help provide a control strategy that adjusts combustion quality in future engine cycles to allow for improved performance of the engine. Thus HCCI-type engines benefit from more accurate and robust determination of SOC and other indicators of combustion quality.
Aside for the HCCI example provided above, knowledge of the combustion quality, such as start and rate of combustion, is becoming commercially beneficial for diesel and SI engines if for no other reason than to monitor performance of the system for real time diagnostic purposes. Corrective actions can be taken with such engines to compensate for measured changes in combustion quality to maintain high engine fuel economy and low engine out emissions.
A known technique for estimating the combustion quality of an engine cycle relies on direct pressure measurement within the combustion chambers; for example, by positioning a sensor to measure the deflection of a diaphragm in contact with the in-cylinder pressure. The measured pressure signal is correlated to a SOC or other combustion quality indicator. For example, a feedback control loop is used to adjust engine parameters to influence the SOC in future engine cycles by minimizing the error between the measured SOC and a target SOC. See U.S. Pat. No. 6,598,468 and German Patent No. 4341796.5 which describe direct measurement of a signal indicative of pressure within the combustion chamber. Typically, an optical sensor, or other direct pressure measurement instrument, is used. While fairly accurate indicators of combustion quality, such sensors are expensive and/or currently lack the reliability and robustness (due to the harsh environment within a combustion chamber) required for many applications.
Another technique for estimating the combustion chamber pressure uses a knock sensor (accelerometer), such as described in U.S. Pat. No. 6,408,819. While accelerometers tend to be less expensive, and currently more reliable and more robust, than direct pressure measuring sensors, a drawback is that this technique relies on a method of reconstructing a pressure signal that is not sufficiently accurate for many combustion quality control methods including effective SOC control.
A more effective system using accelerometers is described in the applicant's co-pending U.S. application Ser. No. 10/822,333 (“'333”) filed Apr. 12, 2004, entitled “Method And Apparatus For Controlling An Internal Combustion Engine Using Accelerometers”, and incorporated by reference herein in its entirety. Rather than reconstructing a pressure signal from the accelerometer data, the '333 application describes a heat release rate reconstruction (HRR) method to extract combustion information from raw accelerometer data.
It was investigated whether the effectiveness of the above mentioned technique could be improved.